Safe-arm mechanism for explosive trains



March 1967 J. HENNESSY ETAL 3,308,757

SAFE-ARM MECHANISM FOR EXPLOSIVE TRAINS Filed June 30, 1965 INVENTOR5 James Hennessy George A.Nodd' nited States Patent 3,308,757 SAFE-ARM MECHANISM FOR EXPLOSIVE TRAINS James Hennessy, Pitman, and George A. N oddin, Mantua,

N.J., assignors to E. I, du Pont de Nemours and Company, Wilmington, DeL, a corporation of Delaware Filed June 30, 1965, Ser. No. 468,423 6 Claims. ((11. 102-22) This invention relates to arming systems for explosive trains and, more particularly, to arming systems which are disarmed in a gaseous medium such as air but armed when immersed in a liquid such as water.

For activities in which the detonation of explosive charges below the surface of water is desired, such as in seismic exploration, underwater signaling, echo-ranging for detection purposes, and destruction of undersea craft, there is a need for a reliable and accurate explosive train which can be armed, i.e., made capable of propagating an initiation impulse, when submerged, yet will be safe, i.e., disarmed by virtue of being incapable of propagating a detonation stimulus, under conditions of storage or removal from the water or other liquid.

Commonly, mechanisms for precluding accidental actuation of all components of explosive trains comprising a donor charge, and a receptor charge, in explosive assemblies intended for underwater use depend upon physical separation of the components by maintaining these ele ments out of line or separated by a gap sufficient to preclude actuation of the receptor charge, should accidental actuation of the donor charge occur. To arm such an explosive train, mechanisms are provided to move the components into position, i.e., nearer to one another or into line, so that continuous detonation of the explosive train will occur upon actuation of the donor charge. This type of arming mechanism generally requires the use of complex mechanical systems which, in order to function properly, must be in prescribed alignment and adjusted to move the elements smoothly from the safe to the armed positions. However, the requisite alignment and adjustment are diflicult to insure since the mechanisms are susceptible to damageduring handling and launching of the explosive assembly containing the explosive train. Further, in such mechanisms, the arming is often irreversible, thus once armed such assemblies must be fired.

Recently, an arming system has been proposed wherein the donor charge and the receptor charge in an explosive train are separated by a fixed distance sufiicient to preclude actuation of the acceptor charge by the donor charge when the two are in air but to allow propagation of a detonation impulse from the donor to the receptor charge when the two are in a liquid, i.e., in water. The train is armed by changing the medium in the space separating the donor and receptor charges, i.e., the train is armed by replacing the air, normally occupying this space in the assembly before the train is armed, by a liquid, i.e., water, by immersing the train in the liquid or by pumping liquid into the space. This system, which is described in US. 3,065,694, is based on the theory that shock waves are conducted more efficiently by a liquid, e.g., water, than by a gaseous medium, e.g., air. Although such a system is attractive in that it would minimize or eliminate the need for complex arming mechanisms, the theory upon which the systems is based does not prove valid or practicable for commercially available explosive charges. Rather, it has been determined experimentally, using commercially available explosive charges and accessories, that a detonation impulse is not propagated as readily between spaced apart donor and receptor charges when the train is in a liquid, i.e., water, as when the train is in air. For example, when a A-inch long piece of detonating cord containing a central core of lead azide at a loading of 34 grains per foot is used in a thin-bottomed tubular aluminum cap 3,368,757 Patented Mar. 14, 1967 shell as the donor charge and the receptor charge is 4 grains of PETN pressed into a thin-bottomed tubular aluminum cap shell (0.240 inch bottom thickness), a spacing of at least A inch must be provided to preclude actuation of the receptor charge by the donor charge when the two are in air. However, when water occupies the space between the charges, the detonation impulse cannot be propagated from the donor charge to the receptor charge over this distance. In fact, when the space is occupied by water, the spacing must be inch or less to allow propagation from the donor to the receptor charge. Thus, an arming mechanism in which the donor and receptor charges of an explosive train are separated by a distance sufficient to preclude propagation of a detonation impulse in air would still preclude propagation when the charges are in water, i.e., the train would not be armed in water.

Accordingly, a need still exists for an arming system for underwater use in which the arming mechanism is not dependent upon the interaction of mechanical parts but is positive and reliable in functioning. Such a system should be reliably operational at both shallow and great depths and should be easily incorporated into the explosive train.

In accordance with this invention, it has been found that certain metal-sheathed detonating fuses will not propagate a detonation stimulus in a gaseous medium such as air if two spaced apart sections along its length are brought together to form a loop and then twisted or wrapped about each other to form an entwined segment at the base of the loop, rather the two sections are blown apart without the detonation stimulus being propagated from one section to the other. However, when immersed in a denser medium, i.e., a liquid medium such as oil or water, the detonation stimulus is propagated without hiatus through the length of thus looped and entwined fuse.

The present invention provides, therefore, a safe-arm mechanism characterized by its ability to propagate a detonation stimulus when surrounded by a liquid medium and its inability to propagate such a stimulus when surrounded by a gaseous medium and which comprises a length of detonating fuse in the form of a loop with an entwined segment at its base, said detonating fuse having a core of cap-sensitive, high-velocity detonating explosive at a loading of about from 5 to 25 grains/foot encased in a ductile metal sheath, and the ratio of said core loading to sheathing thickness (inches) being about from 250 to 700.

This invention also provides an arming system wherein an exposed safe-arm mechanism, as described above, connects a donor charge (e.g., an initiator) to a receptor charge (e.g., a base, booster, or primer charge) which is spaced from the donor charge at least a distance suflicient to preclude sympathetic actuation of the receptor charge by the donor charge.

The term exposed is used herein with reference to the looped and entwined detonating fuse to denote that at least the entwined segment is neither enclosed nor confined in such a manner that it is incapable of being contacting and surrounded by the arming liquid medium.

The looped and entwined fuse is particularly preferred for providing a safe-arm mechanism which is safe when in contact with air, yet armed when immersed in water. It is particularly effective for use with uncountered detonating fuse having a core of high velocity, cap-sensitive detonating explosive, preferably encased in lead sheathing, at a core loading of about from 5 to 25 grains/foot and a ratio of coreloading in grains/foot t-o sheathing thickness, in inches, of about from 250 to 700. The length of fuse within the loop will usually be about from 1 to 10 inches and the entwined segment at its base will be provided by at least 1.5 full twists or wraps, i.e., of about 360, in the fuse.

In order to. describe the invention in greater detail,

reference is now made to the accompanying drawings wherein:

FIGURES 1 and 2 are views of arming systems provided with arm-safe mechanisms in accordance with this invention.

In the drawings in which like reference characters designate like or corresponding parts, D designates the donor charge and R designates the receptor charge in an explosive train designed to be used underwater. Extending between charges D and R is a length of detonating fuse F comprising a core of high-velocity, cap-sensitive detonating explosive in a sheath of a ductile metal, preferably lead. Segment S is provided as a safe-arm mechanism. In FIG- URE 1, segment S is composed of sections AA and BB of the fuse which are entwined, i.e., twisted together, and loop A-B connects these entwined sections.

In FIGURE 2, there is shown an arming system containing an unidirectional safe-arm mechanism which will only stop, i.e., cut-off, a detonation stimulus propagated in the direction of the arrow in air. In the safe-arm segment S of this explosive train, the entwined segment of fuse is provided by forming a loose knot of the halfhitch type in the fuse. With this configuration, the length of fuse in loop section A-B can be slightly less (e.g., about less) than that required for the configuration of FIGURE 1.

The arming systems of this invention are in an inert or safe condition in air during their normal storage and handling since a detonation stimulus will not be propagated from the donor to the receptor charge in air. The detonation stimulus is cut off or dissipated in the loop and entwined segment of fuse since detonation of the explosive core in one section is sufiiciently brisant in air or other gaseous medium to shatter and scatter the section of fuse with which it is in contact. Consequently, the detonation stimulus proceeds to and through the looped portion of fuse connecting the sections but its return path back through the entwined segment of fuse leading to the receptor charge is destroyed usually at point B. When the explosive train or assembly is in a fluid medium, typically water or oil, the confinement presented by the denser medium precludes shattering and scattering of the entwined sections of the fuse through which a detonation stimulus is proceeding. Accordingly, the detonation stimulus passes through one entwined section into and through the looped portion of the fuse, and returns through the other entwined section to the portion of the fuse extending between segment S and the receptor charge.

The detonating fuse employed in the safe-arm means of this invention comprises a core of high velocity 1200 meters/see), cap-sensitive detonating explosive, usually at a core loading of about 5 to about grains per foot. Examples of such explosives are pentaerythritol tetranitr-ate (PETN), cyclotrimethylenetrinitramine (RDX), cyclotetramethylenetetranitramine (HMX), tetryl, tetranitrodibenzo 1,3a,4,6a (or 1,3a,6,6a)-tetraazapentalene, TNT, nitromannite, picryl sulfone, bis(trinitroethyl)urea and mixtures of one or more of the foregoing explosives. Explosives such as those just mentioned can be used alone or in combination with one or more additives, e.g., conventional graining agents and binders, provided the core leading exclusive of such additives is as described above. A preferred core composition for many under-water applications from the viewpoint of water-resistance, as well as dimensional stability and flexibility over a wide range of conditions of temperature and pressure, is of a composition of the type described in US. Patent 2,992,087, which comprises at least 44 weight percent of superfine PETN in combination with a binder of nitrocellulose and a trialkyl ester of 2-acetoxy-1,2,3-propanetricarboxylic acid. Another particularly preferred core composition is that described in U.S. Patent 2,999,743 comprising 92.5 to 70 weight percent of high explosive and 7.5 to percent of a binder comprising 25 to 75 percent of an organic rubber, e.g., butyl rubber, and 75 to 25 percent of a thermoplastic terpene hydrocarbon resin. Other suitable compositions for use in the core of the detonating fuse include those described in copending patent application Ser. No. 437,555 filed March, 5, 1965, which comprise RDX or HMX in a plasticized nitrocellulose binder.

The core of explosive composition will be contained in a sheath of a ductile metal such as lead or aluminum. The thickness of sheathing required to allow one section to shatter another with which it is in contact in a gaseous medium such as air but to preclude this shattering or rupture under the confinement presented by a denser medium such as water or oil will, of course, depend upon the brisance and ease of initiation of the explosive composition comprising the core and the strength of the material chosen as the sheathing. More brisant and more easily initiated explosives require sheathings of greater thickness or constructed of a stronger and preferably more ductile material to preclude shattering in a liquid medium. Usually, to prevent shattering in a liquid medium such as oil or water, the ratio of detonating explosive in the core, in grains/foot exclusive of additives and/or binders, to wall thickness of a sheathing such as lead, in inches, will be about from 250 to 700, and preferably will be about from 260 to 525.

Retaining means such as tape, a clip, or other type holder which may have a channel to receive the entwined sections of fuse may be provided to maintain the required entwining and loop in the fuse where rough handling is anticipated. However such retaining means should not preclude free communication between this section of fuse and the surrounding medium whether this medium be air or a fiuid such as water or oil.

The loop formed between the two sections of fuse will be of generally circular or elliptical configuration, and its circumference will vary with the core loading of the fuse and its detonation velocity, larger loops being employed with higher core loadings and detonation velocities. The size will be chosen such that cut-off from one section (B-B) by the other (A-A') will occur be fore the detonation impulse propagated from section A-A' through the loop returns through section BB'. As a rule, the length of fuse in the loop will be chosen so that a minimum of about from 5 to 40 microseconds is allowed for cut-off of the detonation in fuses having core loadings of about from 5 to 25 grains/foot, respectively. This will generally correspond to a minimum loop length of about from 1 to 8 inches, respectively.

The entwining at the base of the loop provides contact between spaced sections of the fuse and thus ensures that one section will shatter and scatter the other section thereby causing a hiatus in the explosive train. The entwining is rovided by twisting spaced sections of cord about each other as in FIGURE 1 or wrapping such sections about each other, e.g., by forming a knot as in FIGURE 2. Preferably the spaced sections are twisted about each other as in FIGURE 1 so that there will be at least 1.5 twists of about 360. Generally there will be no advantage in having more than about 3 of these twists.

The distance between the donor charge and the receptor charge must be such that the detonation impulse produced by the donor charge will not initiate the receptor charge. For some applications, it may be desirable to have this distance sufficient to preclude actuation of either charge by the other. As a practical matter, the distance between the charges will normally be chosen in accordance with the air gap-sensitivity of the receptor charge; however, if the environment of the explosive train is to be another gaseous medium, the sensitivity of the receptor charge in that medium Will determine the minimum distance between this charge and the donor charge.

If desired, the safe-arm mechanism can be mounted in a housing having means, e.g., apparatus which can be covered by water-soluble disks, for bringing liquid into contact with the looped and entwined segment of detonating fuse. Also, the housing can be provided with a reservoir for fluid and a conventional two-way pump, which can be actuated by a timing mechanism provided to pump fluid to and from the chamber surrounding the segment of detonating fuse.

air and in water (5 tested in each medium), five out of five tested cut-off a detonation stimulus in air but in water in five out of five the detonation stimulus is propagated from the donor to the receptor charge.

A variety of means may be used to initiate detonation in 5 As mentioned earlier, the arming system of this inventhe donor charge. A pressure-actuated initiator, e.g., 3. tion or the Segment of fuse formed into the Safe-arm detonator, is particularly suitable since actuation of the mechanism, can be protected y a Shell Provided it p donor charge will occur only in a liquid at a predetermits the arming fluid to contact and surround at least mined depth, i.e., pressure. However, actuation of the the entwined SeetiOhS 0f the fuse- Meetns can p donor charge can also be initiated by an initiator having 10 Vided regulating the how of aiming fluid into the a conventional electric ignition assembly or an electric chamber 50 that Positive arming of the explosive train delay ignition assembly fired by a bridgewire in a loose can be regilitlted in accordance With the needs of a P ignition composition, a bridgewire and bead arrangement, tieuial Situation, Positive arming 0f the device can an exploding bridgewire, or an arc firing system, in which be deieyed to insure Safety of the arming device of to case the lead wires of the electric ignition assembly exiheide With actuation of a hydrostatic Pressure-Pumai tend to a source of electric current at the surface of the ihitiator- The P e of Water other liquid medium water and the sink rate of the explosive assembly must 111 Sufiicieht q y to cothPieteiy Surround the entwined be known Alternatively, the donor charge can he aetu. sections of detonating fuse will cause arming of the device ated by an electric detonator fired by a fluid actuated batregardless of the depth 0f the liquid medium in which tery, e.g., a sea-battery and pressure-sensitive switch which the eXBiOSiVe train is Submetged- The Simple exchange will fi the detonator at a predetermined depth of liqu1ds, e.g., water or oil, for air, within the chamber The following examples illustrate specific embodiments eliminates the necessity of Complicated mechanisms of the inventiom moving parts and provides an arming system which is reliable and nonhazardous. The explosive train can be Example 1 disarmed or rendered safe should recovery of the device Explosive trains are set up in which a sheet of the exbe desired simply by replacing the liquid in the chamber truda'ble explosive composition described in US. 2,992,- by a gas s medium. It is to be understood, however, O87 (commercially available as Detasheet C or EL- that a protective shell need not be used and the explosive 506-C containing 63%, by weight, of PETN) is used as train may comprise simply blocks or slabs of explosive the receptor charge as shown in FIGURE 1. The recepcharge as the donor and acceptor charge maintained at tor charge in each array measures 2 x 2 x A in. and a predetermined spacing, e.g., by a support means, and Weighs 24 grams. The donor charge is an electric ected by the detonating fuse. detonator of the type commercially available as an E-94 We claim: and has a base load of 2 grains of PETN. The charges A11 arming System Comprising a donor rg a are separated by a distance of 4 inches, sufiicient to pre- C P I Cha g parated from the donor charge by disclude sympathetic detonation of one charge by influence tauee Suiiieieut to preclude Symptithetic actuation of the of the other in air and thus water. The two charges are receptor charge y a detonation Stimulus from the nor connected by a length of detonating fuse composed of a Charge, and a Safe-arm mechanism Connecting Said lead sheathed core of the extrudable explosive composition Charges, d as th receptor h d h l h of f i 40 said safe-arm mechanism being characterized by its provided with a safe-arm segment resembling that of ability to Propagate a detonation us to the FIGURE 1. The characteristics of the cord, the length receptor Charge When Surrounded y e iiquid medium of loop A-B and the number of full twists of fuse at the and its inability to Propagate Such a l s n base of the loop in each of the various arrays are shown Surrounded y a gaseous medium and comprising in the table. Five units of each type are tested in air a length of detonating fuse in the form of a p and when submerged under water. In the table, F inwith all entwined Segment at its Said detonatdicates that the receptor charge is not actuated even ihg e having eeefe of cap-Sensitive, high-Velocity though the donor charge (the detonator) functions propdetohatlug eXPiOSiVe at a loading of about from 5 erly. D indicates that the receptor charge is actuated, to 25 grains/feet encased in a duetiie Iuetfli Sheath, i.e., that a detonation stimulus is propagated from the 50 and the ratio of Said core loading to Sheathing thickdonolto h receptor charge ness in inches being about from 250 to 700.

TABLE Core Core Functioning Lading Detonation Fuse Di- Wall Loading] Length of Full of Ex- Velocity, ameter Thickness, Sheathing Loop A-B, Twists plosive, m./sec. (inches), inches Thickness inches In In gin/ft. O.D. Air Water 5 5, 500 0. 072 o. 019 263 4 2 5F 5D 10 6,250 0.105 0. 027 370 1. 5 1. 5 5F 5D 10 6, 250 0. 0. 027 370 1. 5 2 5F 5D 10 6, 250 0.081 0. 015 667 2 2 5F 5D 13 e, 400 0. 0. 032 406 2 1. 5 5F 5D 19 6, 900 0. 0. 039 488 6 3 5F 5D 22 7, 100 0. 162 0. 042 524 10 3 5F 5D Example 2 2. An arming system of claim 1 wherein the distance Explosive trains are set up using receptor and donor 70 between the donor and receptor charges is suflicient to charges as described in Example 1. An unidirectional proclude said sympathetic actuation in-bot-h air and water, safe-arm means is provided in these explosive trains by and said gaseous medium is air and said liquid medium is tying a knot of the half-hitch type shown in FIGURE 2 water. at the base of a 3-inch loop of the 5 gr./ft.-fuse employed 3. An arming system of claim 1 wherein the loop and in Example 1. When the explosive trains are tested in 75 said entwined segment comprise a half-hitch knot.

4. An arming system of claim 1 wherein said entwined References Cited by the Examiner segment is provided by twisting spaced sections of the UNITED STATES PATENTS detonating fuse about each other at least 1.5 times.

5. An arming system of claim 4 wherein the length of 30958l2 7/1963 course 102' 27 fuse in said loop is about from 1 to 10 inches, the entwined 5 31169481 2/1965 Stresau et 102 27 segment is composed of 1.5 to about 3 full twists, and I said Sheath is lead. BENJAMIN A. BORCHELT, Plzmary Examine].

6. An arming system of claim 5 wherein the ratio of V. R. PENDEGRASS, Assistant Examiner. core loading to sheathing thickness is about from 260 to 525. 

1. AN ARMING SYSTEM COMPRISING A DONOR CHARGE, A RECEPTOR CHARGE SEPARATED FROM THE DONOR CHARGE BY DISTANCE SUFFICIENT TO PRECLUDE SYMPATHETIC ACTUATION OF THE RECEPTOR CHARGE BY A DETONATION STIMULUS FROM THE DONOR CHARGE, AND A SAFE-ARM MECHANISM CONNECTING SAID CHARGES, SAID SAFE-ARM MECHANISM BEING CHARACTERIZED BY ITS ABILITY TO PROPAGATE A DETONATION STIMULUS TO THE RECEPTOR CHARGE WHEN SURROUNDED BY A LIQUID MEDIUM AND ITS INABILITY TO PROPAGATE SUCH A STIMULUS WHEN SURROUNDED BY A GASEOUS MEDIUM AND COMPRISING ALENGTH OF DETONATING FUSE IN THE FORM OF A LOOP WITH AN ENTWINED SEGMENT AT ITS BASE, SAID DETONATING FUSE HAVING A CORE OF CAP-SENSITIVE, HIGH-VELOCITY DETONATING EXPLOSIVE AT A LOADING OF ABOUT FROM 5 TO 25 GRAINS/FOOT ENCASED IN A DUCTILE METAL SHEATH, AND THE RATIO OF SAID CORE LOADING TO SHEATHING THICKNESS IN INCHES BEING ABOUT FROM 250 TO
 700. 